In the field of mass spectrometry (MS), over the past two decades, the use of electrospray ionization (ESI) has grown rapidly, particularly for biological applications. Its use has been accompanied by efforts to increase the ESI-MS sensitivity since only a small fraction of the analyte ions ever reach a mass spectrometer detector. Most ion losses can be attributed to incomplete droplet desolvation and/or poor transport from the atmospheric pressure region to the high vacuum region of a mass analyzer.
Two of the important factors affecting ionization efficiency, thus ESI-MS sensitivity, are the solution flow rate and the mode of electrospray operation. By reducing the solution flow rate, smaller droplets that are more readily desolvated can be formed. Accordingly, it can be advantageous to deliver the electrospray ionization solution to an ESI emitter at the lowest practical flow rate. Operation of the electrospray in the stable “cone-jet” mode, as opposed to other electrospray operation modes (e.g., pulsating, dripping, astable, etc.), can help to ensure that droplets are uniformly small, rather than a mixture of large and small droplets.
ESI emitter arrays, which include a plurality of individual emitters, can have the potential to provide a relatively high total solution flow rate while maintaining the lowest practical flow rate in each emitter. However, electrical shielding effects, which are not necessarily uniform among emitters in the array, can disrupt the cone-jet mode of operation in certain ones, though not necessarily all, of the emitters. The shielding can be caused by electrostatic interference between neighboring emitters. Therefore, in one example, the emitters in the outer portions of the array can experience a higher electrical field than those closer to the center. For a given applied voltage, the outermost emitters might experience corona discharge, the innermost emitters might operate in pulsating mode, and only a portion might operate in cone-jet mode. Furthermore, regardless of specific spray modes, ESI-MS sensitivity is significantly influenced by the electric field, and a particular field can exist that will provide maximum sensitivity. For example, there are many combinations of emitter geometries, flow rates, and solvents for which cone-jet mode operation is impossible. However, a maximum sensitivity will still be observed at a particular electric field, and will get worse as the field is either increased or decreased. So, even when cone-jet mode is not attained, a non-uniform field further contributes to decreased performance. Accordingly, a need exists for improved ESI emitter arrays, and particularly those operating at very low flow rates.